Security Paper Comprising Windows

ABSTRACT

The invention relates to a security paper for the production of documents of value which comprises a flat cellulose-containing substrate with a polymeric window included therein, to processes for the production of a security paper of this type and to the use thereof for the production of documents of value.

The present invention relates to a security paper for the production of documents of value, where the paper comprises a flat cellulose-containing substrate with at least one polymeric window included therein, the polymeric window and/or the cellulose-containing substrate preferably have an optically variable appearance, and the security paper preferably has improved mechanical stability and improved tear strength. The invention furthermore relates to processes for the production of a security paper of this type and to documents of value which comprise a security paper of this type.

Documents of value and security documents, such as, for example, banknotes, passports, identity documents, shares, bonds, certificates, cheques, credit notes, entry tickets, travel tickets, security labels and the like, are often made from paper or materials which have at least one layer of a cellulose-containing material.

In order to increase the counterfeiting security, documents of this type are provided with a multiplicity of security features. In particular in the case of documents of value which are produced in large quantities, for example banknotes, the type and number of the desirable security features must be balanced against the increased production costs. For this reason, there is a constant search for inexpensive and technically simple solutions which can if possible achieve a multiple benefit and significantly increase the counterfeiting security. Optimal solutions are those which result in security features which can be recognised and checked by anyone, if possible without aids, and can be combined well with the known security features that are frequently employed.

A particularly preferred security feature in documents of value is a more or less transparent window in a layer of a document of value, through which it is possible to see a security feature, which, in the case of a multilayered document of value, is present in an underlying layer or which itself represents a validatable security feature through at least one integrated security element which can be perceived visually or by machine.

Thus, AU Patent 488,652 describes a document of value which consists of two or three polymer layers, one of which carries an optically variable feature which can be perceived and checked through at least one transparent polymer layer lying on top. The layer lying on top protects the optically variable security feature and cannot be changed or removed without destruction. However, since the entire document of value consists of polymeric materials, it is only of limited suitability, in particular for the production of banknotes, since polymeric banknotes of this type cannot be provided with security features, such as, for example, portrait watermarks and window security threads, although these are popular as banknote security features and are frequently used. At the same time, the effect of the steel engraving often employed on paper banknotes for printing relief-like print designs is greatly weakened since only very flat relief prints can be produced by this type of printing on a pure polymer substrate, meaning that the special haptic properties of this security feature are partly lost.

Attempts have therefore also already been made to provide cellulose fibre-containing banknote papers with polymeric windows in order to be able to combine the properties and advantages of paper with those of a polymeric window. To this end, paper sheets must be provided with cut-outs, with the paper subsequently either being coated over the entire surface with polymer layers or alternatively only the cut-out being adhesively bonded locally to a polymer layer. Both solutions have disadvantages.

Since the cut-out window openings in the paper sheets generally have a not inconsiderable size for practical reasons, the paper sheets provided therewith tend to lose shape when subjected to mechanical processes. In the mass production of, for example, banknotes, however, mechanical stresses of this type are usual. If it is intended to laminate paper sheets provided with cut-outs with polymer layers over the entire surface, varying window shapes, cracks or creasing may therefore occur.

In the case of the coating of the paper sheets over part of the surface merely at the window openings, these are usually adhesively bonded to film pieces. The thickness of the security paper at the window opening is therefore usually greater than the thickness of the paper as such, and problems occur on stacking of the security paper since the paper sheets do not lie flat on one another.

Accordingly, there was a great demand for the provision of a security paper which has a polymeric window without the disadvantages described above.

In addition, papers for use in the security area must have high mechanical strength. In particular, banknotes are subjected to large mechanical and environmental stresses. The circulation life of banknotes is accordingly frequently determined by their degree of soiling.

In particular due to the principal use of cotton fibres, banknote papers are very porous and therefore have a high tendency towards soiling in circulation.

In order to increase the circulation life, it has therefore been proposed to provide banknote papers with coatings which are intended to reduce the pick-up of dirt by the papers.

Thus, DE 198 29 004 A1 describes a security paper which has on at least one of its surfaces a coating which consists merely of a binder. This layer is said to form a continuous surface film on the surface of the paper, minimising access of dirt to the fibre. Binders which can be employed are acrylates or polyurethanes.

It is likewise known to provide security papers with polymeric layers which are intended to provide the paper with additional strength and water-repellent properties, as described, for example, in EP 1 115 948 B1.

DE-A 2 307 894 also discloses a process in which plastic-containing papers are produced by adding polymeric materials to the paper stock. However, the suspension used must contain particles having sizes of 4 to 30 μm in order that they can be taken up by the paper fibres during the papermaking process in order to provide the paper with strength.

Core/shell particles have also already been described for use in papers. Thus, DE 197 27 060 A1 describes a process for the preparation of coarse aqueous polymer dispersions which are said to be suitable for the finishing of paper. The properties that the papers treated therewith have were not described.

EP 0 441 559 A2 discloses core/shell particles which have a cavity between core and shell and can likewise be used for papermaking. These provide the paper treated therewith with hiding power, brightness and lustre and can replace part of the additives that are otherwise usual, such as kaolin or titanium dioxide.

In order to tint papers, corresponding dyes in particulate or dissolved form are either introduced into the paper pulp or applied via the sizing. This enables paper either to be uniformly coloured or provided with functional dyes, for example with photoluminescent coloured pigments. However, an optically variable appearance cannot be achieved by the use of optically variable pigments in the paper pulp since the paper fibres at least partly cover the pigments and hinder their alignment.

The object of the present invention was to provide a security paper which has a polymeric window which is present in the same layer as a cellulose-containing substrate used for the production of the security paper without projecting beyond this, where at least one of the cellulose-containing substrate and polymeric window constituents preferably has a security feature which can easily be identified visually without aids, and the security paper preferably simultaneously has increased tear strength, good mechanical stability, a low soiling tendency and a tactile surface which is clearly different from untreated paper and can be produced by a simple process which can readily be integrated into the conventional papermaking process.

The object of the present invention is achieved by a security paper for the production of documents of value, which comprises a flat cellulose-containing substrate with at least one polymeric window included therein.

The polymeric window here is opaque, semitransparent or transparent, but preferably transparent or semitransparent, i.e. it preferably transmits incident light to the extent of at least 10% of the incident amount of light.

The object of the invention is likewise achieved by a process for the production of a security paper in which core/shell particles which have a shell of polymeric material are introduced into an aqueous cellulose-containing paper pulp and, together with further paper raw materials, converted into a paper sheet, and the paper sheet is provided with at least one cut-out for a window, in which elevated pressure or elevated pressure and elevated temperature are allowed to act on the paper sheet in such a way that at least some of the core/shell particles present in the paper sheet are pressed into the cut-out so that the core/shell particles fill the cut-out, and in which the shell of the core/shell particles forms a matrix on the paper sheet, at least in the cut-out and in an edge zone between cut-out and paper sheet.

In addition, the object of the invention is also achieved by a process for the production of a security paper in which core/shell particles which have a shell of polymeric material are introduced into an aqueous cellulose-containing paper pulp and, together with further paper raw materials, converted into a paper sheet, and the paper sheet is provided with at least one cut-out for a window, and in which further core/shell particles having a polymeric shell are applied to the cut-out in the paper sheet so that the further core/shell particles fill the cut-out, and in which elevated pressure or elevated pressure and elevated temperature are allowed to act on the paper sheet in such a way that the shell of the core/shell particles forms a matrix on the paper sheet, at least in the cut-out and in an edge zone between cut-out and paper sheet.

In addition, the object of the invention is achieved by a process for the production of a security paper in which core/shell particles which have a shell of polymeric material are applied to at least part of the surface of an unsized or sized paper, in which the paper has or is provided with at least one cut-out for a window, and in which elevated pressure or elevated pressure and elevated temperature are allowed to act on the paper in such a way that at least some of the core/shell particles present in or on the paper are pressed into the cut-out so that the core/shell particles fill the cut-out, and in which the shell of the core/shell particles forms a matrix on the paper, at least in the cut-out and in an edge zone between cut-out and paper.

The object of the invention is furthermore also achieved by the use of the above-mentioned security paper for the production of documents of value, such as banknotes, passports, identity documents, shares, bonds, certificates, cheques, credit notes, entry tickets, travel tickets, security labels and the like, and by the provision of documents of value of this type.

Like other papers, security papers are produced in a papermaking machine in which the following working steps are generally carried out successively: stock preparation, stock processing, the wire section, the press section, the dryer section, surface finishing, smoothing, and cutting.

Stock production here serves principally for producing the cellulose-containing starting material for papermaking. This can be obtained from various vegetable fibres or also from rags. Security paper is preferably produced using cotton fibres, which can be obtained either directly from cotton plants, but also from rags.

In the pulper, the various paper ingredients, which consist of the cellulose-containing paper raw material and various additives, are mixed with water to give a paper suspension, the pulp. The additives here are selected so that they influence a very wide variety of desired properties of the paper, such as colour, smoothness, whiteness, weight per unit area, strength, water-repellent properties, etc., but may also contain particles or fibres which already provide the finished security paper with security features, such as, for example, planchettes (small paper or plastic platelets), fibres of various materials (for example plastics), which may also, inter alia, have photoluminescent properties, fluorescent starlets, chemical additives which exhibit specific chemical reactions or chemical reactions which can be detected with the aid of special light sources, and the like.

In the wire section, the highly diluted aqueous paper suspension is distributed uniformly on a circulating wire screen, where excess water runs off or is removed by suction. True watermarks are also introduced into the paper in this wire section.

The excess water is removed in the press section, and the consolidated paper web formed is dried in the dryer section under the action of heat.

In the surface finishing which usually follows, the paper is subjected to a so-called sizing or coating process, which generally reduces the sorbency of the paper. This sizing is usually carried out with binders and/or pigments and serves to produce the desired surface properties, such as weight per unit area, relative moisture content, toner adhesion and fixing, porosity, pH, lustre, whiteness and the like.

This is followed by a smoothing process, in which the paper web is passed through a plurality of rolls, and finally paper cutting.

It can be seen from the process sequence roughly outlined here that, during papermaking, elevated pressure and elevated temperature act a number of times on the paper raw materials or on the paper web forming. The raw materials and additives employed in papermaking have to withstand this temperature and pressure load in order to be able to achieve the desired effects, unless the changes in the stock properties caused by pressure and temperature produce precisely the desired effects.

The security paper in accordance with the present invention comprises a cellulose-containing substrate, which is also referred to below as paper or paper sheet and which consists of the usual materials for the production of security papers, i.e. preferably comprises cellulose from vegetable fibres and/or rags and in particular cellulose fibres from cotton. In addition, the cellulose-containing substrate may likewise comprise plastic fibres (also predominantly plastic fibres) and further conventional additives. The choice of additives here is dependent on the desired paper properties and can vary greatly. For the purposes of the present invention, the nature of the additives is not crucial and is therefore not limiting so long as they do not react chemically with the core/shell particles introduced into the paper pulp in accordance with the present invention or applied to or introduced into the cellulose-containing substrate, in such a way that they change the optical properties thereof. To this extent, the expert knowledge of the papermaker will determine what additives he adds to the production process for the production of the security paper according to the invention.

The cellulose-containing substrate is preferably a sized or unsized paper.

The cellulose-containing substrate has a polymeric window, i.e. a cut-out in the cellulose-containing substrate is filled with a material which has entirely or predominantly polymeric constituents. The polymeric window here is preferably located in a plane with the cellulose-containing substrate and does not project beyond either the lower or upper surface of the cellulose-containing substrate. In particular, the polymeric window is connected intimately and in an adherent manner to the cellulose-containing substrate without being adhesively bonded or laminated thereto.

The polymeric window in the cellulose-containing substrate is, in accordance with the invention, a moulding of core/shell particles which have a polymeric shell.

In particular, the polymeric window comprises core/shell particles whose cores are essentially solid and dimensionally stable and have an essentially monodisperse size distribution.

If it is intended to form an optically variable appearance, shell material and core material must have a difference between their refractive indices.

At least in the polymeric window and in an edge zone between the cut-out for the window and the cellulose-containing substrate (paper), the polymeric shell of the core/shell particles forms a matrix on the paper. In this matrix, the cores are preferably arranged regularly, i.e. they form three-dimensional structures by means of which a long-range order of the cores is achieved, which approximately corresponds, at least from domain to domain, to cubic face-centred spherical closest packing.

If there is a refractive index difference between the shell material and the core material of the core/shell particles, the regularly arranged cores form a diffraction grating at which reflection, interference and scattering of the incident light occur simultaneously. This gives the moulding of core/shell particles an opalescent colouring.

The polymeric window can therefore preferably have an optically variable appearance, which as such represents an independent security feature. At the same time, the polymeric window can have polarising-filter properties.

The cellulose-containing substrate for the security paper in accordance with the present invention also comprises, at least distributed over part of its surface, core/shell particles which have a polymeric shell. Core/shell particles of this type are present at least in an edge zone between window cut-out and cellulose-containing substrate, but preferably also on further parts of the surface or on the entire surface of the cellulose-containing substrate.

The core/shell particles here are likewise preferably those whose cores are essentially solid and dimensionally stable and have an essentially monodisperse size distribution.

It is likewise advantageous here if there is a difference between the refractive indices of the core material and of the shell material.

Due to the conventional papermaking process, in which elevated pressure or elevated pressure and elevated temperature act on the paper sheet, and/or due to aftertreatment with elevated pressure or the action of elevated pressure and temperature, at least some of the core/shell particles present in the cellulose-containing substrate are also present in domains with regularly arranged cores, where the latter form a diffraction grating if core material and shell material have different refractive indices. Incident light is reflected, interfered with and scattered thereby, which causes an optically variable appearance of the cellulose-containing substrate.

In a preferred embodiment, the cellulose-containing substrate or the polymeric window, but preferably both, therefore has optically variable properties in accordance with the invention. In addition, the polymeric window may have light-polarising properties and act as a polarising filter.

In general, the term optically variable is applied to visually perceptible properties in which a different colour and/or brightness impression is evident at different illumination and/or viewing angles. In the case of different colour impressions, this property is referred to as colour flop. A security feature having a property of this type exhibits colour and lustre impressions which are readily perceptible with the naked eye, but cannot be copied.

The substrate and/or the polymeric window in the security paper according to the invention preferably have at least two and at most four visually clearly distinguishable discrete colours at at least two different illumination and/or viewing angles, but in particular two visually clearly distinguishable discrete colours at two different illumination and/or viewing angles or three visually clearly distinguishable discrete colours at three different illumination and/or viewing angles. A colour progression which occurs on tilting via different illumination and/or viewing angles represents a further embodiment. Both colour changes can readily be registered by the human eye and cannot be copied.

The cellulose-containing substrate and/or also the polymeric window preferably have a certain degree of transparency. With respect to the polymeric window, this means that it should transmit at least 10% of the incident light. A special feature of the security paper according to the invention comes to light here. If the cellulose-containing substrate and/or the polymeric window have a certain optically variable colour combination, for example a colour flop from violet to blue-green, when viewed from the top, the complementary, likewise optically variable colour combination, here, for example, from yellow-green to orange, is observed when viewed through it.

This colour play, which is more intense in the polymeric window than in the cellulose-containing substrate, cannot be mimicked using the conventional optically variable means for the production of security products and cannot be copied using conventional colour copiers. It therefore represents an independent, conspicuous security feature which can easily be verified visually without aids.

Even if the polymeric window or cellulose-containing substrate is not transparent, the colour play perceptible when viewed from the top is sufficiently conspicuous to represent an independent security feature.

Between the core/shell particles which are present in the polymeric window and the core/shell particles which are present in an edge zone around the polymeric window in the cellulose-containing substrate, a common matrix of the shell components of the core/shell particles is formed, which ensures a strong connection of the moulding of core/shell particles in the polymeric window to the cellulose-containing substrate without the need for adhesive-bonding or lamination processes between paper and polymer.

This common matrix can of course extend over a larger part or alternatively over the entire surface of the security paper according to the invention, but is formed at least in the edge zone between the cellulose-containing substrate and the polymeric window consisting of a moulding of core/shell particles.

The core/shell particles present in the cellulose-containing substrate and in the polymeric window may be identical or different. The variation possibilities here relate both to the chemical composition of the base materials and additives for core and shell, the nature of the chemical bonding of the shell to the core, the particle size of the core/shell particles and also the weight distribution of core and shell.

The core/shell particles in the cellulose-containing substrate and in the polymeric window are preferably identical since this can be achieved using a very simple process and the core/shell particles can be added in a single process step. At the same time, clear optically variable effects can thus already be obtained in substrate and window.

For security products with a more complex design, however, it is also advantageous for cellulose-containing substrate and polymeric window to have different colour combinations and/or different machine-readable constituents. This can be achieved by variation of the above-mentioned parameters. Selective application of core/shell particles to the cut-out for the window can likewise be achieved technically without major effort.

The cores of the core/shell particles preferably have an essentially spherical, in particular ball-like shape and have an essentially monodisperse size distribution, i.e. they have a very narrow particle-size distribution.

The average particle diameter of the core particles is in the range 30-400 nm, in particular in the range 60-350 nm and particularly preferably in the range 90-300 nm. In general, the particle diameter of the core particles is about 60 to about 80%, in particular about 65 to about 75%, of the total diameter of the core/shell particles.

The core/shell particles have an average particle diameter in the range from about 50-800 nm. In particular, particles in the range 100-500 nm are employed and particularly preferably particles having a particle diameter of 150-400 nm. In these particle-size ranges, optical effects in the visible wavelength region of light can preferably be expected.

However, it is also possible to employ core/shell particles whose size corresponds to a multiple of the particle sizes described here.

The cores of the core/shell particles are essentially solid and dimensionally stable. This means that the cores either do not become flowable under the processing conditions in the papermaking process or during production of the core/shell particles or become flowable at a temperature above the melting point of the shell material. Under the same conditions, the material of which the cores consist is also virtually non-swellable.

In order to achieve this, the core materials selected are preferably organic polymeric materials having a correspondingly high glass transition temperature (T_(g)) or alternatively inorganic core materials.

The cores preferably consist of or predominantly comprise an organic polymeric material which is, in particular, crosslinked.

Both polymers and copolymers of polymerisable unsaturated monomers and also polycondensates and copolycondensates of monomers having at least two reactive groups, such as, for example, high-molecular-weight aliphatic, aliphatic/aromatic or fully aromatic polyesters, polyamides, polycarbonates, polyureas and polyurethanes, but also amino resins and phenolic resins, such as, for example, melamine-formaldehyde, urea-formaldehyde and phenol-formaldehyde condensates, are suitable. Epoxy resins are also suitable as core material.

In a preferred variant of the invention, the polymers of the core material are advantageously crosslinked (co)polymers since these usually only exhibit their glass transition at high temperatures. These crosslinked polymers may either already have been crosslinked in the course of the polymerisation or polycondensation or copolymerisation or copolycondensation, or they may have been post-crosslinked in a separate process step after completion of the actual (co)polymerisation or (co)polycondensation.

The monodisperse cores of organic polymeric materials are preferably obtained by emulsion polymerisation. Regarding the course of this process and all assistants and additives used, such as, for example, polymerisation initiators, dispersion aids, emulsifiers, crosslinking agents and the like, express reference is made here to the corresponding comments in EP 0 955 323 A1 and in WO 03/025035 A2.

In another, likewise preferred variant of the invention, the core consists entirely or predominantly of an inorganic material, preferably a metal or semimetal or a metal chalcogenide or metal pnictide.

For the purposes of the present invention, chalcogenides are taken to mean compounds in which an element from group 16 of the Periodic Table of the Elements is the electronegative bond partner; pnictides are taken to mean those in which an element from group 15 of the Periodic Table of the Elements is the electronegative bond partner.

Preferred cores consist of metal chalcogenides, preferably metal oxides, or metal pnictides, preferably nitrides or phosphides. For the purposes of these terms, metals are all elements which are able to occur as electro-positive partner compared with the counterions, such as the classical sub-group metals or the main-group metals from the first and second main groups, but equally also all elements from the third main group, as well as silicon, germanium, tin, lead, phosphorus, arsenic, antimony and bismuth. The preferred metal chalcogenides and metal pnictides include, in particular, silicon dioxide, aluminium oxide, gallium nitride, boron nitride, aluminium nitride, silicon nitride and phosphorus nitride.

The starting material employed for the production of the core/shell particles in a variant of the present invention is preferably monodisperse cores of silicon dioxide, which can be obtained, for example, by the process described in U.S. Pat. No. 4,911,903. The cores here are produced by hydrolytic poly-condensation of tetraalkoxysilanes in an aqueous/ammoniacal medium, where firstly a sol of primary particles is produced, and the resultant SiO₂ particles are subsequently brought to the desired particle size by continuous, controlled metered addition of tetraalkoxysilane. This process enables the production of monodisperse SiO₂ cores having average particle diameters between 0.05 and 10 μm with a standard deviation of 5%.

The starting material is also preferably SiO₂ cores which have been coated with (semi)metals or metal oxides which do not absorb in the visible region, such as, for example, TiO₂, ZrO₂, ZnO₂, SnO₂ or Al₂O₃. The production of SiO₂ cores coated with metal oxides is described in greater detail, for example, in U.S. Pat. No. 5,846,310, DE 198 42 134 and DE 199 29 109.

The starting material employed can also be monodisperse cores of non-absorbent metal oxides, such as TiO₂, ZrO₂, ZnO₂, SnO₂ or Al₂O₃, or mixtures of metal oxides. The preparation thereof is described, for example, in EP 0 644 914. The process of EP 0 216 278 for the production of monodisperse SiO₂ cores can furthermore readily be applied to other oxides with the same result. Tetraethoxysilane, tetrabutoxytitanium, tetrapropoxyzirconium or mixtures thereof are added in one portion with vigorous mixing to a mixture of alcohol, water and ammonia whose temperature is precisely set to 30 to 40° C. using a thermostat, and the resultant mixture is stirred vigorously for a further 20 seconds, during which a suspension of monodisperse cores in the nanometre region forms. After a post-reaction time of 1 to 2 hours, the cores are separated off in a conventional manner, for example by centrifugation, washed and dried.

Also suitable as starting material for the production of the core/shell particles are monodisperse cores of polymers which comprise included particles which consist, for example, of metal oxides. Materials of this type are offered, for example, by micro caps Entwicklungs-und Vertriebs GmbH in Rostock. Microencapsulations based on polyesters, polyamides and natural and modified carbohydrates are manufactured in accordance with customer-specific requirements.

It is furthermore possible to employ monodisperse cores of metal oxides which have been coated with organic materials, for example silanes. The monodisperse cores are dispersed in alcohols and modified using standard organoalkoxysilanes. The silanisation of spherical oxide particles is also described in DE 43 16 814.

The size and particle-size distribution of the cores can be set particularly well if the cores consist predominantly or exclusively of organic polymers and/or copolymers. The cores preferably consist predominantly of a single polymer or copolymer and particularly preferably of polystyrene.

The cores of the core/shell particles may likewise comprise a contrast agent. This can be a soluble or insoluble colorant. Soluble colorants are generally soluble, usually organic dyes, which can be of natural or synthetic origin and are generally selected from the compound classes of the carbonyl colorants, such as quinones, indigoid colorants and quinacridones, the cyanine colorants, such as di- and triarylmethanes and quinonimines, the azo colorants, the azomethines and methines, the isoindoline colorants, the phthalocyanines and the dioxazines. Insoluble colorants are organic or inorganic coloured pigments. These are preferably absorption pigments and, in a variant of the invention, particularly preferably black pigments, for example carbon black.

However, these contrast agents are usually inorganic or organic pigments, which can be of natural or synthetic origin. For the purposes of the present invention, pigments are taken to mean any solid substance which exhibits an optical effect in the visible wavelength region of light or has certain functional properties. In particular, the term pigments is applied to substances which conform to the definition of pigments in accordance with DIN 55943 or DIN 55944. According to this definition, a pigment is an inorganic or organic, coloured or uncoloured colorant which is virtually insoluble in the application medium or a substance which has particular properties, for example magnetic, electrical or electromagnetic properties, and is virtually insoluble in the application medium. The shape of these pigments is not important, in particular they can be of a spherical, flake-form or needle-shaped nature or have irregular particle shapes.

It goes without saying that pigments which are incorporated into the cores of the core/shell particles have an average particle size which is not greater than the average particle size of the cores.

The contrast agents employed in the cores can also be luminescent compounds. Luminescent compounds are taken to mean substances which emit machine-measurable and optionally visible radiation due to excitation in the visible wavelength region, in the IR or UV wavelength region of light, due to electron beams or due to X-rays. These also include substances which emit radiation due to excitation in an electromagnetic field, so-called electroluminescent substances, which optionally additionally luminesce due to excitation in the UV or IR wavelength region. Suitable for this purpose are all known particulate and soluble substances having the above-mentioned properties. The particulate substances here can have any suitable regular or irregular shape and have a suitable particle size, i.e. have an average particle size which does not exceed the average particle diameter of the cores. The luminescent particles are therefore particularly preferably in the form of nanoparticles or in the form of so-called quantum dots. The particulate substances do not necessarily have to be in pure form, but instead may likewise encompass microencapsulated particles and support materials impregnated, doped or coated with luminescent substances. For this reason, luminescent substances can be incorporated into the cores or as cores of the core/shell particles. This relates both to soluble and also particulate luminescent materials.

Besides organic luminescent substances of any type, examples which may be mentioned of luminescent substances are, for example, the following compounds: Ag-doped zinc sulfide ZnS:Ag, zinc silicate, SiC, ZnS, CdS, which has been activated by Cu or Mn, ZnS/CdS:Ag; ZnS:Cu, ZnS:Tb; ZnS:Al; ZnS:TbF₃; ZnS:Eu; ZnS:EuF₃; Y₂O₂S:Eu; Y₂O₃:Eu; Y₂O₃:Tb; YVO₄:Eu; YVO₄:Sm; YVO₄:Dy; LaPO₄:Eu; LaPO₄:Ce; LaPO₄:Ce,Tb; Zn₂SiO₄:Mn; CaWO₄; (Zn,Mg)F₂:Mn; MgSiO₃:Mn; ZnO:Zn; Gd₂O₂S:Tb; Y₂O₂S:Tb; La₂O₂S:Tb; BaFCl:Eu; LaOBr:Tb; Mg tungstate; (Zn,Be)-silicate:Mn; Cd borate:Mn; [Ca₁₀(PO₄)₆F, Cl:Sb, Mn]; (SrMg)₂P₂O₇:Eu; Sr₂P₂O₇:Sn; Sr₄Al₁₄O₂₅:Eu; Y₂SiO₅:Ce, Tb; Y(P,V)O₄:Eu; BaMg₂Al₁₀O₂₇:Eu or MgAl₁₁O₁₉:Ce,Tb. This list is merely illustrative and should therefore not be regarded as final.

Magnetic particles whose average particle diameters do not exceed the average particle diameters of the cores of the core/shell particles can likewise be incorporated into the cores of the core/shell particles. This is readily possible, in particular, if an organic polymer is used as core material. In principle, all magnetic particles which consist of magnetisable materials or comprise magnetisable materials as the core, coating or doping are suitable for this purpose. Magnetisable materials which can be employed here are all known materials, such as magnetisable metals, magnetisable metal alloys or metal oxides and oxide hydrates, such as, for example, γ-Fe₂O₃ or FeOOH. Their usability is determined merely by the average particle size, which must not be greater than the average particle size of the cores. Their shape is not important here, in particular needle-shaped magnetic particles may also be incorporated.

For the purposes of the invention, fibrous or particle-shaped additives which are essentially transparent and colourless are also to be regarded as contrast agents. These are preferably particles or fibres of plastics, glass or other solid, transparent materials which are different from the core material and are introduced into the core material in order to increase the mechanical strength of the core/shell particles.

As for the core material, suitable polymers for the polymeric shell material are in principle those from the classes already mentioned above, so long as they have been selected or built up in such a way that they conform to the specification given for the shell polymers, namely preferably become flowable due to an increase in pressure or in pressure and temperature.

If optically variable effects are to be achieved, the shell material has a refractive index which is different from the refractive index of the core material. This determines that the core and shell cannot simultaneously consist of the same material.

It is not important here whether the core or shell has the higher refractive index. However, it is advantageous for the core to consist of a material having a higher refractive index than the shell material. In order to achieve a clear optically variable effect, it has been found that the difference between the refractive indices of the core and shell material should be at least 0.01 and in particular at least 0.1.

In order to achieve attractive optically variable effects in the polymeric window and, if desired, also in the cellulose-containing substrate, the shell material must be filmable, while the cores remain solid and dimensionally stable. This applies at least to the shell material of the core/shell particles which are present in the polymeric window and in the surrounding edge zone of the cellulose-containing substrate. This means that the shell material can be heated to a temperature at which the shell is flowable. This flowability can also be achieved by the action of elevated pressure alone or alternatively by the action of elevated pressure and elevated temperature. In the process, the shell is softened, viscoelastically plasticised or liquefied. The shell material here has a melting point which is significantly lower than the melting point of the core material.

In a particular embodiment of the present invention, core and shell do not have refractive index differences which result in the formation of optically variable effects. Instead, core and shell can be constructed, with an essentially identical refractive index, in such a way that the cores remain solid and dimensionally stable under the flow conditions of the shell (pressure and temperature) and are provided with one of the contrast agents described above, causing the latter to be uniformly distributed in the matrix of the moulding of core/shell particles and to result in properties which can be evaluated visually and/or by machine and which are readily evident, in particular, in the polymeric window. These properties can be, for example, a certain colour, electrical conductivity, magnetic properties, luminescent properties or the like. However, if only one particular uniform detectable property is desired, for example colour, the cores of the core/shell particles do not have to remain solid and dimensionally stable under the flow conditions of the shell, but instead may likewise become at least partially flowable and thus essentially only serve to introduce the contrast agents into the polymeric window and/or the cellulose-containing substrate in a simple manner. In this case, a somewhat non-uniform distribution of the contrast agents is acceptable. Also in the latter case, a polymeric matrix forms which forms a polymeric window and connects the latter to the cellulose-containing substrate in an adherent manner.

In the simplest embodiment of the present invention, the shell material is already softened in such a way that it becomes filmable by the action of pressure or pressure and temperature during the conventional papermaking process.

In a further embodiment, the shell material is only softened in such a way that it becomes filmable by the action of elevated pressure or by the action of pressure and heat in a pressing and/or embossing process in a process step following the papermaking process.

However, it is likewise advantageous for the shell material already to have been softened by the application of pressure or pressure and temperature during the conventional papermaking process, where the degree of softening can be further increased and the filmability of the material thus improved by subsequent pressing and/or embossing processes.

Polymers which meet the specifications for the shell material are likewise in the groups of the polymers and copolymers of polymerisable unsaturated monomers, and also polycondensates and copolycondensates of monomers having at least two reactive groups, such as, for example, high-molecular-weight aliphatic, aliphatic/aromatic or fully aromatic polyesters and polyamides.

Taking into account the above conditions for the properties of the shell polymers, selected building blocks from all groups of organic film formers are in principle suitable for the preparation thereof.

Some further examples may illustrate the broad range of polymers which are suitable for the preparation of the shells.

If the shell is to have a comparatively low refractive index, polymers such as polyethylene, polypropylene, polyethylene oxide, polyacrylates, polymethacrylates, polybutadiene, polymethyl methacrylate, polytetrafluoroethylene, polyoxymethylene, polyesters, polyamides, polyepoxides, polyurethane, rubber, polyacrylonitrile and polyisoprene and copolymers thereof, for example, are suitable.

If the shell is to have a comparatively high refractive index, polymers having a preferably aromatic basic structure, such as polystyrene, polystyrene copolymers, such as, for example, SAN, aromatic-aliphatic polyesters and polyamides, aromatic polysulfones and polyketones, polyvinyl chloride, polyvinylidene chloride, and, given a suitable choice of a high-refractive-index core material, also polyacrylonitrile or polyurethane, for example, are suitable for the shell.

The shell materials employed can also be elastically deformable polymers, such as, for example, various polyurethanes, low-molecular-weight polyesters, silicones or polyether- or polyester-modified silicones.

Like the cores, the shells of the core/shell particles may also comprise a contrast agent. Essentially all contrast agents which have already been described above for uptake into the cores of the core/shell particles are suitable here. In contrast to the uptake of the contrast agents into the cores, the particulate contrast agents are not, however, subject to any significant size restriction on incorporation into the shells. Instead, solid, particulate contrast agents whose particle sizes are significantly larger than the average particle diameters of the core/shell particles themselves can also be incorporated into the shells of the core/shell particles. This is attributable to the fact that the shell materials employed have a clear “adhesion tendency” with respect to foreign particles. The shape of the insoluble contrast agents employed is not restricted either on incorporation into the shells of the core/shell particles, instead contrast agents in any suitable shape can be employed.

In addition to the contrast agents, assistants and additives which are not of a particulate nature, for example flow improvers, dispersion aids, emulsifiers and the like, can also be incorporated into the shells of the core/shell particles.

The cores of the core/shell particles used in accordance with the invention are preferably chemically bonded to the shell via an interlayer. This means that the cores are modified in such a way that bonding of the shell takes place via chemical bonds, but not by simple physical attachment. They are preferably covalent bonds. In certain cases, however, electrostatic bonding of the shell to the core is also sufficient.

In a preferred embodiment of the invention, the interlayer is a polymeric interlayer, for example a layer of crosslinked or at least partially crosslinked polymers. The crosslinking of the interlayer can take place here via free radicals, for example induced by UV irradiation, or preferably via di- or oligofunctional monomers. Preferred interlayers in this embodiment comprise 0.01 to 100% by weight, particularly preferably 0.25 to 10% by weight, of di- or oligofunctional monomers. Preferred di- or oligofunctional monomers are, in particular, isoprene and allyl methacrylate (ALMA). Such an interlayer of crosslinked or at least partially crosslinked polymers preferably has a thickness in the range from less than 1 nm to 20 nm. If the interlayer is thicker, the refractive index of this layer is selected so that it either corresponds to the refractive index of the core material or to the refractive index of the shell material.

If copolymers which, as described above, contain a crosslinkable monomer are employed as interlayer, the person skilled in the art is presented with absolutely no problems in suitably selecting corresponding copolymerisable monomers. For example, corresponding copolymerisable monomers can be selected from a so-called Q-e scheme (cf. textbooks of macromolecular chemistry). Thus, monomers such as methyl methacrylate and methyl acrylate can preferably be polymerised with ALMA.

In another, likewise preferred embodiment of the present invention, the shell polymers are grafted directly onto the core via corresponding functionalisation of the core. The surface functionalisation of the core forms the above-mentioned interlayer. The type of surface functionalisation here depends principally on the material of the core. Silicon dioxide surfaces can be suitably modified, for example, by means of silanes carrying correspondingly reactive end groups, such as epoxy functions or free double bonds. Other surface functionalisations, for example for metal oxides, can take place with titanates or organoaluminium compounds, which in each case contain organic side chains having corresponding functions. In the case of polymeric cores, a styrene functionalised on the aromatic ring, such as bromostyrene, can be employed, for example, for the surface modification. The growing-on of the shell polymers can then be achieved via this functionalisation. In particular, the interlayer can also effect adhesion of the shell to the core via ionic interactions or complex bonds.

In a preferred embodiment, the shell of the core/shell particles consists of essentially uncrosslinked organic polymers, which have preferably been grafted onto the core via an at least partially crosslinked interlayer.

The shell here can either consist of thermoplastic or of elastomeric polymers. Since the shell essentially determines the material properties and the processing conditions of the core/shell particles, the person skilled in the art will select the shell material in accordance with usual considerations in polymer technology.

The interlayer in the core/shell particles employed in accordance with the invention guarantees stability of the core/shell particles to the influence of elevated pressure and elevated temperature, which ensures that no phase separation of core and shell occurs under these conditions. This is particularly important if optically variable effects are desired. By contrast, the structure of core/shell particles whose shell is merely placed against the core generally cannot be retained under the action of elevated pressure and elevated temperature. Exertion of pressure, in particular, will in this case result in the shell material being separated from the core material and consequently the optical effect previously achievable by the different refractive indices of core and shell being eliminated.

The weight of the shell in the core/shell particles employed in accordance with the invention is preferably equal to or greater than the weight of the core. The core:shell weight ratio is particularly preferably in the range from 1:1 to 1:5, in particular in the range from 1:1 to 1:3 and particularly advantageously in the range from 1:1.1 to 2:3, i.e. the weight of the shell is preferably greater than the weight of the core. This core:shell weight ratio is a preferred feature of the present invention. In particular, the proportion by weight of the shell must be sufficiently large in order to facilitate, via the large number of polymer chains thus present, that the core/shell particles can be retained on the fibrous paper raw materials in the papermaking process, even in the case of particle sizes which are low overall, and are not removed from the paper suspension by the wire.

Furthermore, the comparatively high proportion by weight of the shell is the prerequisite for the core/shell particles employed in accordance with the invention being able to arrange themselves in a substantially regular structure during drying and smoothing of the paper substrate since the polymeric shell material usually already softens to a certain degree under the usual papermaking conditions and is at least partially converted into a film within the fibre structure of the paper.

Since the core/shell particles can only be admixed in a limited amount with the paper stock, a significantly lower proportion by weight of the shell would, by contrast, not result in the formation of a film phase at all, at least in the case of the core/shell particles present in the cellulose-containing substrate.

Core/shell particles which are suitable for the security paper in accordance with the present invention can be produced, for example, in accordance with the examples given in WO 03/025035.

The core/shell particles described above are present in the cellulose-containing substrate and in the polymeric window in a first embodiment of the security paper in accordance with the present invention.

For this purpose, the core/shell particles, preferably in the form of a predominantly aqueous dispersion, can be admixed with the usual starting materials for papermaking. As already described above, these include the cellulose-containing paper raw material and the various additives. Depending on the desired paper properties, these are selected expertly by the papermaker and are only limited inasmuch as they must not undergo any chemical reactions with the above-mentioned core/shell particles which change the optical properties of the core/shell particles.

Additives which are suitable for the formation of independent security features in the finished security paper, for example planchettes, fibres of various materials, photoluminescent fibres, photoluminescent particles, such as, for example, fluorescent starlets, or chemical additives which can be detected with the aid of special light sources or exhibit specific chemical reactions, may also already be admixed with the paper suspension prepared from the starting materials. In the same way, magnetic or electrically conductive substances may be present.

In a first variant of the invention, the polymeric window is obtained by allowing elevated pressure or elevated pressure and elevated temperature to act on the substrate comprising the core/shell particles in such a way that at least some of the core/shell particles are forced out of the substrate into at least one window cut-out which is located on the substrate and that a matrix in which the cores are preferably uniformly distributed forms at least there and in the surrounding edge zone of the shells of the core/shell particles. This is described in greater detail below.

In a second embodiment of the invention, core/shell particles are present in the cellulose-containing substrate and, due to the papermaking process, form, at least in domains, a matrix with cores regularly included therein. The substrate is provided with at least one cut-out for a window. Further core/shell particles, which may have a different composition from the core/shell particles present in the cellulose-containing substrate or have the same composition, are applied to this cut-out. In this way, the properties of substrate and window which can be evaluated visually and/or by machine can be varied. The core/shell particles present in the window cut-out are treated with elevated pressure or elevated pressure and elevated temperature as described above so that the shell forms a matrix. Since the core/shell particles applied also penetrate into the edge zone on the paper adjacent to the cut-out, a matrix which intimately connects the core/shell particles in the window to the cellulose-containing substrate in an adherent manner forms there together with the particles in the window.

The core/shell particles employed in accordance with the invention preferably provide the security paper and/or the polymeric window with an optically variable character, which is attributable to the formation of regular structures of the cores both in the cellulose-containing substrate and also in the polymeric window. At the same time, the polymeric window may have polarising-filter properties.

It has not been definitively explained, but it is assumed that the mere exertion of pressure and temperature under the usual conditions in the papermaking machine is sufficient to soften the shell of the core/shell particles sufficiently that the shell material at least partially forms a matrix in the paper in which the cores are able to arrange themselves regularly.

The achievable optical effect here is crucially determined by the refractive-index difference of the core and shell materials and by the particle diameter of the cores.

If an optically variable appearance of the security paper is desired, it goes without saying that the difference in the refractive indices of core and shell should be as large as possible since this enables the clearest possible visible optically variable effects to be obtained. This is achieved, for example, in the case of the choice of polystyrene as core polymer and polyethyl acrylate as shell polymer, giving a refractive-index difference of 0.12. However, material combinations whose refractive-index difference is smaller are also suitable. These result in opalescent effects, which are likewise optically variable.

The addition of core/shell particles of various size and composition enables the optically variable colouring of the security paper according to the invention to be steered in a simple manner, for example if different colours are desired for different denominations of banknotes, without the papermaking process having to be modified in other components or process steps.

It is particularly advantageous that both identical and also different optical and/or machine-readable properties can be set specifically for substrate and polymeric window. A large variation latitude of measurable properties can thus be obtained at only very low cost.

The optically variable properties of the security paper according to the invention can also be enhanced subsequently, for example by subsequent pressure or temperature and pressure treatment. In particular by pressing and/or embossing processes over part of the surface, specific effects can thus be produced at predetermined points of the security paper. Thus, for example, the true watermarks present in the paper are characterised in that the paper layer is particularly thin at these points. If core/shell particles are present in the paper material, a specific embossing process at the point of the watermark enables the latter to be emphasised in a particularly transparent and at the same time optically variable manner. Here too, the effect already described occurs that the optically variable colours that are perceptible in incident light are replaced in transmitted light by the likewise optically variable complementary colours. This measure results in the watermark, as probably the best known security feature in paper or paper-like materials, obtaining an optically variable colour design and thus experiencing a considerable enhancement, both optically and in terms of security.

An enhancement of the optically variable effect and the transparency can likewise be observed if the polymeric window is subsequently subjected to an additional pressing and/or embossing treatment. It has also proven particularly advantageous that the polymeric window can be provided with a mark directly by laser irradiation without further additives being necessary. In this way, it is possible to obtain a polymeric window which simultaneously has two different visible security features, namely an optically variable effect and a laser mark.

At the same time, the addition of the core/shell particles to the security paper according to the invention achieves increased mechanical strength of the paper, in particular increased tear strength, and improved water-repellent properties of the security paper. The porosity of the security paper likewise decreases, enabling a reduced tendency towards soiling to be noted. The tactile properties of the security paper in accordance with the present invention likewise improve. The addition of the core/shell particles gives it a so-called “soft touch”, i.e. the surface of the security paper feels very soft and smooth, but not purely paper-like. Depending on the amount of added core/shell particles, tactile surface properties can thus be obtained which can be ascribed neither to pure paper nor to pure polymer film and combine the surface properties of the two materials. The added amount of core/shell particles also determines the degree of “film-likeness” of the paper, i.e. if the added amount is increased, the visible and tactile paper properties decrease and the visible and tactile film properties increase.

In a third embodiment of the present invention, the core/shell particles are present on the cellulose-containing substrate, where the latter has or is provided with at least one cut-out for a window. This can be carried out by the introduction of core/shell particles into the conventional sizing layer, by the application of a dispersion of core/shell particles instead of the conventional sizing layer or by the application preferably of a dispersion of core/shell particles to a sizing layer that has already been applied. The application of these layers can take place both over the entire surface or over part of the surface of the cellulose-containing substrate, enabling targeted control of the areas on which the optically variable effect achieved by the core/shell particles is visible. It is thus possible to apply the core/shell particles, for example, only to the window cut-out, to coat certain part-areas of the cellulose-containing substrate, for example at the level of the watermark, or alternatively to carry out a full-area coating. The formation of the matrix takes place as already described above.

If a sizing layer is present, this may, irrespective of whether the core/shell particles are present therein or not, comprise all ingredients which are otherwise usual in papermaking, such as pigments, binders and the like, so long as they do not react with the core/shell particles in such a way that they adversely affect the optical properties thereof.

If the core/shell particles are incorporated into the conventional sizing layer or are applied to the finished cellulose-containing substrate instead of a sizing layer, this layer seals at least some of the pores present on the surface of the cellulose-containing substrate and thus penetrates into the substrate to a certain degree.

As already described above for the introduction of the core/shell particles into the paper stock, these also provide the security paper according to the invention with an optically variable appearance, given a corresponding composition, if the core/shell particles are present on the cellulose-containing substrate.

The smoothing process following conventional sizing is generally sufficient here to allow a regular order to form of the cores in a matrix formed from the shell material. Here too, the three-dimensional structures described above, at which reflection, interference and scattering of the incident light occur, are able to form.

Since these structures form better on a paper substrate the less porous the cellulose-containing substrate is, the visible optically variable colour effect is significantly more pronounced in a pre-sized paper than in a paper in which the core/shell particles are present in the sizing or in a layer which replaces the sizing.

However, the more porous the cellulose-containing substrate, the greater an increase in the transparency of this substrate due to addition of the core/shell particles with retention of the optically variable properties. Depending on the desired effect, the person skilled in the art can therefore vary whether he introduces the core/shell particles preferentially into the paper substrate, a layer located directly on the latter or into a coating following the conventional sizing. In the polymeric window, optically variable effects, if they are desired, are already more perceptible than in the cellulose-containing substrate.

In all cases, however, the optically variable effect can be enhanced over the entire surface or over part of the surface by a specific subsequent pressure or temperature and pressure treatment.

The statements already made above regarding the mechanical and tactile properties of the security paper in accordance with the present invention also apply if the core/shell particles are present on the cellulose-containing substrate.

However, the core/shell particles may of course also be present both in and on the cellulose-containing substrate. The optically variable properties of the security paper as well as its film-likeness are thus enhanced.

A major advantage of the security paper according to the invention consists in that it can, in addition to the core/shell particles and the effects associated therewith, contain all conventional security features which are usually used in security papers.

These are not only the security features already described above, such as fluorescent particles or fibres, planchettes, watermarks or the like, which may already be present in the paper stock, but also, for example, security features which are applied to or introduced into the security paper after completion of papermaking, such as security threads, fluorescent dyes, infrared- or UV-active dyes, magnetic particles, electrically conductive particles, optically variable pigments, optically variable layers, optically variable prints, liquid-crystalline coatings, diffractive pigments, holograms, kinegrams, RFID elements, laser marks, chemical additives which become visible on illumination at certain wavelengths or on manipulation, microtexts, guillochés and the like.

These security features are either visible or can be rendered visible by means of aids and/or are machine-readable. An aid of this type may be, for example, the polymeric window according to the invention itself since it has polarising properties.

Preference is therefore given to an embodiment of the present invention in which the security paper, besides the polymeric window and optionally the core/shell particles in the cellulose-containing substrate, additionally has at least one further security feature, in particular one of the security features described above.

The present invention also relates to a process for the production of a security paper in which core/shell particles which have a shell of polymeric material are introduced into an aqueous cellulose-containing containing paper pulp and, together with further paper raw materials, converted into a paper sheet, and the paper sheet is provided with at least one cut-out for a window, in which elevated pressure or elevated pressure and elevated temperature are allowed to act on the paper sheet in such a way that at least some of the core/shell particles present in the paper sheet are pressed into the cut-out so that the core/shell particles fill the cut-out, and in which the shell of the core/shell particles forms a matrix on the paper sheet, at least in the cut-out and in an edge zone between cut-out and paper sheet.

The core/shell particles here are usually introduced into the paper pulp in an amount of about 0.01 to 50% by weight, preferably 1 to 20% by weight, based on the dry weight of the paper.

As already described above, the degree of “film-likeness” of the paper can likewise be controlled by means of the amount of core/shell particles employed, like its surface properties and the optically variable appearance.

The core/shell particles can be introduced into the aqueous paper pulp both in solid form and also in dispersion. Preference is given to addition in the form of a predominantly aqueous dispersion of core/shell particles. Besides water, the dispersion may optionally also comprise various alcohols which are common as solvents.

The papermaking process subsequently proceeds with retention of the conventional process steps. The resultant paper sheet is subsequently provided with cut-outs for windows. This can be carried out both on the still uncut and also preferably on the already cut paper sheet, for example by means of a stamping operation.

The paper sheet provided with at least one cut-out for a window is subjected to treatment under elevated pressure or elevated pressure and elevated temperature. This can be, for example, a rolling, pressing or calendering operation. If elevated temperatures are employed, it is appropriate to set these in accordance with the selected shell material for the core/shell particles so that the melting point of the shell material is reached, so that melt flow processes occur in the shell material. The pressure employed should be at least 1 bar above atmospheric and can be up to 300 bar.

At least some of the core/shell particles evade the pressure acting on the paper sheet and begin to flow, with the window cut-out being filled with core/shell particles. The action of pressure likewise causes the shell of the core/shell particles to form a uniform matrix, in which the cores of the core/shell particles have a regular arrangement, in the cut-out and in an edge zone around the cut-out. This uniform matrix can of course also extend over further regions of the paper sheet or alternatively over the entire paper sheet, with the formation of a diffraction grating by the cores occurring in domains. In the polymeric window, the matrix with the included core particles represents a moulding of core/shell particles. In the case of refractive-index differences between the core material and the shell material, a diffraction grating which results in optically variable colour effects forms. These effects are more pronounced in the polymeric window than in the cellulose-containing substrate since greater uniformity of the diffraction grating formed by the cores can be achieved in the polymeric window.

The action of pressure should take place on a solid, smooth support. Metal surfaces or surfaces of crystalline or partially crystalline polymers are preferably suitable for this purpose since, in particular, the moulding of core/shell particles forming in the polymeric window can easily be detached therefrom as entanglement reactions of the shell polymers with the support do not occur.

In this process for the production of the security paper according to the invention, the core/shell particles present in the cellulose-containing substrate and those present in the polymeric window are inevitably of the same composition and size. Substrate and window thus have the same properties which can be detected visually or otherwise, but which possibly differ in their intensity.

However, the invention also relates to a process for the production of a security paper in which core/shell particles which have a shell of polymeric material are introduced into an aqueous cellulose-containing paper pulp and, together with further paper raw materials, converted into a paper sheet, and the paper sheet is provided with at least one cut-out for a window, and in which further core/shell particles having a polymeric shell are applied to the cut-out in the paper sheet, so that the further core/shell particles fill the cut-out, and in which elevated pressure or elevated pressure and elevated temperature are allowed to act on the paper sheet in such a way that the shell of the core/shell particles forms a matrix on the paper sheet, at least in the cut-out and in an edge zone between cut-out and paper sheet.

As already described above, the core/shell particles here are usually introduced into the paper pulp in an amount of about 0.1 to 10 per cent by weight, based on the dry weight of the paper.

The core/shell particles can be introduced into the aqueous paper pulp either in solid form or in dispersion. The addition is preferably carried out in the form of a predominantly aqueous dispersion of core/shell particles.

As far as the production of a paper sheet provided with a window, this second process proceeds as already described above. Further core/shell particles, preferably in aqueous dispersion, are subsequently applied to the cutout in the paper sheet. This application can take place via a mask, by pad printing or further suitable methods of part-area coating. The nature of the application is not essential to the invention and can be selected from known methods without inventive step.

Through the specific application, the core/shell particles now already fill the cut-out on the paper sheet. Elevated pressure or elevated pressure and elevated temperature are subsequently employed as already described above. This results in the formation of a moulding of core/shell particles which extends at least over the polymeric window and the adjacent edge zone on the paper. However, domains of regularly arranged core/shell particles may also form in the cellulose-containing substrate.

This second process has the advantage that the core/shell particles can be applied directly to the cut-out, and complete filling of the cut-out is thus guaranteed. Furthermore, it is possible for the core/shell particles in the cut-out to have a different structure from the core/shell particles in the cellulose-containing substrate, i.e. they may be different from these. It is thus possible for different properties of substrate and window which can be validated visually and/or by machine to be established in a targeted manner. Thus, for example, it is possible to select different window properties for each of the different denominations of a currency with the same substrate properties, or vice versa. Since the differences do not all have to be evident visually without aids, this enables an extremely wide variety of designs of documents of value, which simultaneously have a high security class.

However, the core/shell particles introduced into the substrate and the core/shell particles introduced into the window may of course also be identical, i.e. have the same composition and size.

The present invention likewise relates to a process for the production of a security paper in which core/shell particles which have a shell of polymeric material are applied to at least part of the surface of an unsized or sized paper, in which the paper has or is provided with at least one cut-out for a window, and in which elevated pressure or elevated pressure and elevated temperature are allowed to act on the paper in such a way that at least some of the core/shell particles present in or on the paper are pressed into the cut-out so that the core/shell particles fill the cut-out, and in which the shell of the core/shell particles forms a matrix on the paper, at least in the cut-out and in an edge zone between cut-out and paper.

The core/shell particles can be applied to the paper at any point of the paper surface, including the cut-out for the window. This application can also take place over part of the surface or only on the window cut-out. It is likewise possible for the core/shell particles to be applied to a paper which has optionally either already been provided with a cut-out for a window or is provided with one after application of the core/shell particles, without core/shell particles likewise being applied to the cut-out. The remainder of the process can be carried out as described above in the first and second process variants.

This third process variant allows most design freedom and thus offers the possibility of coating only the cut-out for the window, this cut-out and parts of the paper or the entire paper with core/shell particles, which may have an identical or different structure. This takes place via simple, optionally also repeated application of core/shell particles to the surface of a paper produced in a conventional process with subsequent pressure or temperature and pressure treatment, as already described above.

Innumerable possible designs of properties which can be read visually and/or by machine can thus be achieved on different part-areas of the security paper in accordance with the present invention.

For the application of the core/shell particles, which is preferably carried out using an aqueous dispersion of these particles, all conventional application techniques, such as, for example, the various printing methods, coating and spreading methods, spraying methods, etc., are suitable. For this purpose, the aqueous dispersions can also be mixed with all suitable solvents, binders or assistants which are usually used for application methods, so long as they do not adversely affect the optical or other evaluatable properties of the core/shell particles.

The core/shell particles here can be applied to the paper surface as a constituent in the conventional sizing layer, as a dispersion of core/shell particles instead of the conventional sizing layer or, preferably in a dispersion, to a sizing layer that has already been applied.

The more porous the paper, the better the core/shell particles penetrate into the substrate. Sized paper has reduced porosity. In the case of pre-sized paper, the core/shell particles therefore preferably remain on the surface of the paper, where they can form, due to the subsequent pressure and temperature treatment, mouldings having a uniform distribution of the cores and thus a regular diffraction grating. For this reason, an optically variable effect, if desired, is obtainable in more pronounced form on a pre-sized paper than on a non-pretreated paper.

The processes described above naturally employ the core/shell particles having a polymeric shell which have already been described in detail above in shape, size, composition and type of bonding of the shell to the core.

Irrespective of whether the core/shell particles are present in or on the cellulose-containing substrate, subsequent pressure treatment or pressure and temperature treatment enables, in particular, the optically variable properties of the security paper according to the invention to be emphasised, the film-like formation of the paper surface or polymeric window to be enhanced or the transparency of the substrate comprising the core/shell particles to be increased.

A subsequent process of this type can be, for example, a smoothing, pressing and/or embossing treatment, which is carried out over the entire surface or part of the surface of the substrate comprising the core/shell particles.

Particularly effective here are embossings, which result in high transparency and a particularly readily visible optically variable effect at the embossed point.

An aftertreatment of this type by pressure or temperature and pressure can be carried out immediately after the paper production. The cellulose-containing substrate here may already contain further security features, such as watermarks, planchettes, fibres, etc. Further security features, such as, for example, security threads, fluorescent dyes, infrared- or UV-active dyes, magnetic particles, electrically conductive particles, optically variable pigments, optically variable layers, optically variable prints, liquid-crystalline coatings, holograms, kinegrams, diffractive pigments, RFID elements, laser marks, chemical additives which become visible on illumination at certain wavelengths or on manipulation, microtexts, guillochés and the like in suitable form, can subsequently be applied to and/or introduced into the cellulose-containing substrate. This is preferably carried out at the points of the substrate where only the conventional, but not subsequent pressure treatment has previously taken place.

However, it is likewise advantageous firstly to apply or introduce further security elements to or into the cellulose-containing substrate comprising core/shell particles or, for example, in the form of prints before a subsequent strengthening pressure or temperature and pressure treatment is carried out. The subsequent pressure treatment here cannot be carried out only over part of the surface, but instead is carried out over the entire surface, producing virtual “sealing” of the other security features since, depending on the proportion of core/shell particles in the cellulose-containing substrate, a film-like surface can form, which may be advantageous, depending on the desired security product.

The present invention furthermore relates to the use of the security paper according to the invention for the production of documents of value of all types, for example for the production of banknotes, passports, identity documents, shares, bonds, certificates, cheques, credit notes, entry tickets, travel tickets, security labels and the like. In principle, all documents of value which are traditionally made from paper or paper-bonded materials (for example laminates), but also documents of value which are traditionally made from plastics, for example ID cards, access authorization documents of all types and the like, can be produced using the security paper according to the invention.

The present invention therefore likewise relates to documents of value produced using the security paper in accordance with the present invention.

A special form of a document of value according to the invention is a document of value which has a security paper with a transparent or semitransparent polymeric window in accordance with the present invention and a second substrate intimately connected thereto, where the latter has at least one security feature and is connected to the security paper according to the invention in such a way that the at least one security feature of the second substrate can be validated visually and/or by machine through the window included in the security paper.

The material of the second substrate is not limited here. Instead, all known materials which are employed for documents of value or security products or themselves form the surface of a product to be protected and can have a security feature which can be detected with or without aids, for example papers, boards, cardboards, plastics, metals or wood, also in the form of multilayered products, such as, for example, laminates, are suitable.

The nature of the security feature that the second substrate posesses is also not limited. Features which are easily detectable visually are preferably employed here, such as optically variable or other prints, photographs, alphanumeric symbols, microtexts, holograms, kinegrams, laser marks and the like, but it is also possible for photoluminescent, electrically conductive, magnetic and other features to be present, optionally also in combination with one another. In addition, a polymeric window in accordance with the present invention can also be employed as polarising filter in order to make security features comprising nematic liquid crystals which are present in an underlying layer visually or machine-readable.

The security paper according to the invention and the second substrate are connected to one another in such a way that at least one security feature of the second substrate can be perceived and validated visually and/or by machine through the polymeric window.

The nature of the connection of the security paper according to the invention to the second substrate is not important here. Any suitable type of connection, for example a permanent or non-permanent adhesive bond, laminates or the like are possible. If the security paper in accordance with the present invention is employed, for example, as security label, it can also be applied directly to the product to be protected, which carries, for example, a bar code, so that the bar code can only be perceived through the polymeric window.

The security paper in accordance with the present invention has a polymeric window which is connected to the paper substrate in an intimate and adherent manner without adhesive bonding or lamination processes being necessary and which is preferably in a plane with the paper substrate and does not project beyond the surfaces thereof. The polymeric window can be opaque, transparent or semitransparent and thus matched in its properties to various possible applications. The polymeric window is preferably transparent or semitransparent and has an optically variable colour combination which, when looked through, exhibits colours which are complementary to the colours which can be perceived when viewed from the top. However, the polymeric window may optionally also have only one optically invariable colour and/or one or more further security features which can be perceived visually or by machine, for example a laser mark.

Due to core/shell particles in and/or on the paper, the cellulose-containing substrate may also have optical or other detectable properties which are identical to or different from those of the polymeric window.

Furthermore, core/shell particles which are present in and/or on the cellulose-containing substrate provide the latter with high mechanical strength, tear strength and water-repellent properties and reduce its tendency to soil rapidly. The substrate here is provided with a surface which differs in tactile terms from a pure paper surface through a particularly smooth, soft touch.

Variation of the composition and size of the added core/shell particles enables, in particular, the optically variable properties of the security paper according to the invention to be controlled specifically in colour and intensity both in the substrate and also in the polymeric window. By contrast, the amount of added core/shell particles influences not only the mechanical and tactile properties of the security paper, but also the degree of achievable film-like properties. Furthermore, the optically variable properties and the transparency of the security paper can be specifically emphasised by subsequent pressure or temperature and pressure treatment.

Straightforward integration of the production process according to the invention into the conventional papermaking process is likewise possible. In addition, the security paper according to the invention may additionally be provided with all conventional further security features which are generally usual for security products.

The security paper according to the invention can preferably be combined with other substrates which likewise have security features so that at least some of the latter are only perceptible through the polymeric window of the security paper according to the invention. This makes copying of such security features more difficult and at the same time also offers mechanical protection against damage or unauthorised removal.

The advantages of the security paper according to the invention, a number of simple processes for the production thereof and multifarious possible uses thereof in documents of value of various types have been explained in detail above.

Its unique structure and its various integratable security elements which can be combined in a variety of ways provide it with a high degree of security, an uncopyable optical appearance and outstanding mechanical properties.

It can therefore be employed with good success both for high-security products and also for the moderate security segment. 

1. Security paper for the production of documents of value, comprising a flat cellulose-containing substrate with at least one polymeric window included therein.
 2. Security paper according to claim 1, where the window is transparent or semitransparent.
 3. Security paper according to claim 1, where the cellulose-containing substrate and/or the polymeric window have optically variable properties.
 4. Security paper according to claim 1, where the polymeric window has light-polarising properties.
 5. Security paper according claim 1, where the polymeric window is a moulding of core/shell particles which have a polymeric shell.
 6. Security paper according to claim 1, where the cellulose-containing substrate comprises core/shell particles which have a polymeric shell.
 7. Security paper according to claim 1, where the shell of the core/shell particles forms a matrix in the window, and the cores are essentially solid and dimensionally stable and have an essentially monodisperse size distribution.
 8. Security paper according to claim 1, where the cellulose-containing substrate comprises core/shell particles whose cores are essentially solid and dimensionally stable and have an essentially monodisperse size distribution.
 9. Security paper according to claim 5, where the core material and shell material of the core/shell particles have different refractive indices.
 10. Security paper according to claim 5, where the core is chemically bonded to the shell via an interlayer.
 11. Security paper according to claim 5, where the weight of the shell is identical to or greater than the weight of the core.
 12. Security paper according to claim 6, where core/shell particles are present in and/or on the cellulose-containing substrate.
 13. Security paper according to claim 1, where the cellulose-containing substrate is a security paper which comprises predominantly cellulose from vegetable fibres and/or rags.
 14. Security paper according to claim 13, comprising cellulose fibres from cotton.
 15. Security paper according to claim 1, where the cellulose-containing substrate is an unsized or sized paper.
 16. Security paper according to claim 5, where the shell of the core/shell particles consists of a material which becomes flowable through an increase in pressure or pressure and temperature.
 17. Security paper according to claim 5, where the core of the core/shell particles consists entirely or predominantly of an organic polymeric material which is either non-flowable or is flowable at a temperature above the melting point of the shell material.
 18. Security paper according to claim 5, where the core of the core/shell particles consists entirely or predominantly of an inorganic material.
 19. Security paper according to claim 5, where the difference between the refractive indices of the core material and shell material is at least 0.01.
 20. Security paper according to claim 19, where the difference between the refractive indices is at least 0.1.
 21. Security paper according to claim 10, where the interlayer is a polymeric interlayer or a surface functionalisation of the core.
 22. Security paper according to claim 5, where the core and/or shell of the core/shell particles additionally comprises at least one contrast agent.
 23. Security paper according to claim 5, where the core/shell particles have particle diameters of 50 to 800 nm.
 24. Security paper according to claim 11, where the core/shell particles have a core:shell weight ratio in the range from 1:1 to 1:5.
 25. Security paper according to claim 11, where the weight of the shell of the core/shell particles is greater than the weight of the core.
 26. Security paper according to claim 1, which has at least one further security feature in addition to the polymeric window.
 27. Security paper according to claim 26, where the additional security features are watermarks, planchettes, fibres, security threads, fluorescent dyes, infrared- or UV-active dyes, magnetic particles, electrically conductive particles, optically variable pigments, optically variable layers, optically variable prints, liquid-crystalline coatings, holograms, kinegrams, diffractive pigments, RFID elements, laser marks, chemical additives which become visible on illumination at certain wavelengths or on manipulation, microtexts, guillochés and the like.
 28. Security paper according to claim 27, where the laser mark is located on the polymeric window.
 29. Process for the production of a security paper according to claim 1, in which core/shell particles which have a shell of polymeric material are introduced into an aqueous cellulose-containing paper pulp and, together with further paper raw materials, converted into a paper sheet, and the paper sheet is provided with at least one cut-out for a window, in which elevated pressure or elevated pressure and elevated temperature are allowed to act on the paper sheet in such a way that at least some of the core/shell particles present in the paper sheet are pressed into the cut-out so that the core/shell particles fill the cut-out, and in which the shell of the core/shell particles forms a matrix, at least in the cut-out and in an edge zone between cut-out and paper sheet.
 30. Process for the production of a security paper according to claim 1, in which core/shell particles which have a shell of polymeric material are introduced into an aqueous cellulose-containing paper pulp and, together with further paper raw materials, converted into a paper sheet, and the paper sheet is provided with at least one cut-out for a window, and in which core/shell particles having a polymeric shell are applied to the cut-out in the paper sheet so that the core/shell particles fill the cut-out, and in which elevated pressure or elevated pressure and elevated temperature are allowed to act on the paper sheet in such a way that the shell of the core/shell particles forms a matrix, at least in the cut-out and in an edge zone between cut-out and paper sheet.
 31. Process according to claim 29, in which the core/shell particles are introduced into the paper pulp in an amount of 0.1 to 50 per cent by weight, based on the dry weight of the paper.
 32. Process according to claim 30, in which the core/shell particles introduced into the paper pulp and the core/shell particles applied to the cut-out are identical or different.
 33. Process according to claim 29, in which a predominantly aqueous dispersion of core/shell particles is employed.
 34. Process for the production of a security paper according to claim 1, in which core/shell particles which have a shell of polymeric material are applied to at least part of the surface of an unsized or sized paper, in which the paper has or is provided with at least one cut-out for a window, and in which elevated pressure or elevated pressure and elevated temperature are allowed to act on the paper in such a way that at least some of the core/shell particles present in or on the paper are pressed into the cut-out so that the core/shell particles fill the cut-out, and in which the shell of the core/shell particles forms a matrix, at least in the cut-out and in an edge zone between cut-out and paper sheet.
 35. Process according to claim 34, in which the core/shell particles are applied to the cut-out in the paper.
 36. Process according to claim 34, in which a predominantly aqueous dispersion of core/shell particles is employed.
 37. Process according to claim 29, in which the security paper is additionally smoothed, pressed and/or embossed over the entire surface or part of the surface.
 38. Process according to claim 29, in which the security paper is firstly provided with various additional security features and subsequently smoothed, pressed and/or embossed over the entire surface or part of the surface.
 39. Process according to claim 38, in which the additional security features are introduced into the cellulose-containing paper pulp and/or applied to and/or introduced into the finished paper.
 40. Process according to claim 38, in which the additional security features are watermarks, planchettes, fibres, security threads, fluorescent dyes, infrared- or UV-active dyes, magnetic particles, electrically conductive particles, optically variable pigments, optically variable layers, optically variable prints, liquid-crystalline coatings, holograms, kinegrams, diffractive pigments, RFID elements, laser marks, chemical additives which become visible on illumination at certain wavelengths or on manipulation, microtexts, guillochés and the like.
 41. (canceled)
 42. Documents of value, such as banknotes, passports, identity documents, shares, bonds, certificates, cheques, credit notes, entry tickets, travel tickets, security labels and the like, comprising a security paper according to claim
 1. 43. Document of value according to claim 42, which has a transparent or semitransparent polymeric window and at least one second substrate, intimately connected thereto, which has at least one security feature, where the second substrate is connected to the security paper in such a way that the at least one security feature of the second substrate can be validated visually and/or by machine through the transparent or semitransparent window included in the security paper. 